Multiplexer/demultiplexer for WDM optical signals

Details Multiplexer/demultiplexer for WDM optical signals Multiplexer/demultiplexer for WDM optical signals

1999-12-16

2002-10-15

Spyrou, Cassandra (Department: 2872)

Optical waveguides

With optical coupler

Input/output coupler

C385S046000, C385S039000, C385S050000, C385S017000

Reexamination Certificate

active

06466715

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a multiplexer/demultiplexer for wavelength division multiplexed (WDM, hereinafter) optical signals, and especially to a multiplexer/demultiplexer for WDM optical signals in which the fluctuation of an insertion loss caused by the fluctuation of a wavelength of an optical signal is slight and the WDM optical signals are multiplexed or demultiplexed with high stability.
BACKGROUND OF THE INVENTION
In the field of the WDM optical communication, it is universally admitted that an arrayed waveguide diffraction grating is promising as a multiplexer/demultiplexer for multiplexing or demultiplexing the WDM optical signals, and various proposals have been made on this subject (Japanese Patent Application Laid-Open Nos.4-116607, 4-1634064, 4-220624, 4-3263084, and 5-157920). Especially, since the arrayed waveguide diffraction grating having a flat passband characteristic hardly shows a variation in an insertion loss caused by the fluctuation of a wavelength of the optical signal and multiplexes or demultiplexes the optical signals with high stability, the engineers in this field place their hopes on this device as one of key technologies in the WDM optical communication (U.S. Pat. No. 5,412,744).
FIG. 1
schematically shows a conventional multiplexer/demultiplexer of an arrayed waveguide diffraction grating type. In this example, nine optical signals having the wavelengths of &lgr;
1
to &lgr;
9
(&lgr;
1
<&lgr;
2
< . . . <&lgr;
8
<&lgr;
9
) are multiplexed or demultiplexed by the device shown in FIG.
1
. For simplicity of explanation, an optical signal having a wavelength &lgr;
i
(i=1, 2, . . . 8, 9) will be expressed by the optical signal &lgr;
i
.
As shown in
FIG. 1
, the conventional multiplexer/demultiplexer for the WDM optical signals are composed of a substrate
201
, an input waveguide
202
, an input slab waveguide
204
, an arrayed waveguide diffraction grating
206
comprising plural channel waveguides
205
having different lengths, an output slab waveguide
207
, and nine output waveguides
208
. The length of the channel waveguide
205
monotonously increases as the position thereof in the arrayed waveguide diffraction grating
206
is high, and the difference in the length between the adjacent channel waveguides
205
is &Dgr;L, which will be explained afterward.
FIGS. 2A
to
2
C schematically shows electric field distributions of the optical signal at important portions in the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating type.
FIG. 2A
shows the electric field distribution
209
of the optical signal at a mode conversion portion
203
in the E-E′ direction,
FIG. 2B
shows the electric field distribution
211
of the optical signal at the input end
210
of the arrayed waveguide diffraction grating
206
in the F-F′ direction, and
FIG. 2C
shows the electric filed distribution
213
at the output end
212
of the arrayed waveguide diffraction grating
206
in the G-G′ direction.
FIGS. 3A
to
3
C schematically show phase distributions of the optical signals at the important portions of the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating.
FIGS. 3A
to
3
C show phase distributions of the optical signals at the output end
212
of the arrayed waveguide diffraction grating
206
in the G-G′ direction.
FIG. 3A
, FIG.
3
B and
FIG. 3C
respectively show the phase distributions
214
,
215
and
216
of the optical signals &lgr;
1
, &lgr;
5
and &lgr;
9
.
FIGS. 4A
to
4
B schematically show differences in the phase distribution between the optical signals at the output end
212
of the arrayed waveguide diffraction grating
206
in the G-G′ direction.
FIG. 4A
show a difference between phase distributions
214
and
215
, those respectively corresponding to the optical signals &lgr;
1
and &lgr;
5
.
FIG. 4B
shows the difference between phase distribution
216
and
215
, those respectively corresponding to the optical signals &lgr;
9
and &lgr;
5
.
FIG. 5
respectively show the electric field distributions
220
,
221
and
222
of the optical signals &lgr;
1
, &lgr;
5
and &lgr;
9
on a focusing surface
219
of the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating type in the H-H′ direction.
Thereafter, the function of the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating type will be explained mainly referring to FIG.
1
. The difference in the length &Dgr;L between the adjacent channel waveguides
205
which constitute the arrayed waveguide diffraction grating
206
is given by the following relation.
&Dgr;
L
=2
m
&pgr;/&bgr;(&lgr;
5
)  (1)
In the equation (1), m is a diffraction order number (a positive integer) and &bgr;(&lgr;
5
) is a propagation constant of the channel waveguide
205
for the optical signal &lgr;
5
.
The optical signals &lgr;
1
to &lgr;
9
supplied to the input waveguide
202
successively propagate through the mode conversion portion
203
, the input slab waveguide
204
, the arrayed waveguide diffraction grating
206
, the output slab waveguide
207
and the output waveguides
208
.
As shown in
FIG. 2A
, the electric field distribution
209
of each optical signal at the mode conversion portion
203
shows a symmetric twin-peak-shaped profile in the E-E′ direction.
As shown in
FIG. 2B
, the electric field distribution
211
of the optical signal at the input end
210
of the arrayed waveguide diffraction grating
206
shows a maximum value and minimum values in the F-F′ direction because of an effect of diffraction. At the input end
210
of the arrayed waveguide diffraction grating
206
, the optical signal is divided, supplied to the respective channel waveguides
205
and propagate therethrough.
As shown in
FIG. 2C
, the electric field distribution
213
of each of the optical signals &lgr;
1
to &lgr;
9
at the output end
212
of the arrayed waveguide diffraction grating
206
in the G-G′ direction is a reproduction of the electric field distribution
211
at the input end
210
of the arrayed waveguide diffraction grating
206
in the F-F′ direction.
As shown in
FIGS. 3A
to
3
C, the phase distributions of the optical signals &lgr;
1
, &lgr;
5
and &lgr;
9
at the output end
212
of the arrayed waveguide diffraction grating
206
in the G-G′ direction are different dependently on the wavelengths of the optical signals. Since the optical signal &lgr;
5
satisfies the equation (1), the phase istribution
215
is symmetric. The phase distributions of the ther optical signals at the output end
212
of the arrayed waveguide diffraction grating
206
incline to the G-G′ direction in accordance with their propagation constants.
As shown in
FIGS. 4A
to
4
B, the difference in the phase destruction between the optical signals &lgr;
1
and &lgr;
5
and the same between the optical signals &lgr;
9
and &lgr;
5
continuously vary at the output end
212
of the arrayed waveguide diffraction grating
206
in the G-G′ direction. In the output slab waveguide
207
, the respective optical signals propagate in the directions corresponding to these inclinations. Accordingly, the optical signals are respectively focussed at different points Y
1
to Y
9
(not shown) on a focussing surface
219
of the output slab waveguide
207
.
As shown in
FIG. 5
, the electric filed distributions
220
,
221
and
222
on the focusing surface
219
in the H-H′ direction are affected by aberration of the output slab waveguide
207
. The electric field distribution
221
of the optical signal &lgr;
5
reproduces the electric field distribution
209
in the mode conversion portion
203
and shows a symmetric twin-peak-shaped profile. On the other hand, the electric field distributions
220
and
222
of the optical signals &lgr;
1
and &lgr;
9
show asymmetric profiles. Since the main cause of asymmetry is aberration, asymmetry becomes noticeab

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